[Quick inquiry, how do you paint a layer mask?]

I have tried to emulate the tutorial on Quickmasks https://player.vimeo.com/video/130973259/.  I can load an image.  Pressing 'q' or clicking on the quickmask icon gives me a red overlay.  I can select the paintbrush tool.  But painting on the image has no effect whatsoever.  Image is attached.  Please explain.  I'm using late 2012 iMac, OS X 10.12.6, Affinity Photo 1.5.2.

IMG_0250.afphoto

[Affinity Photo Customer Beta (1.5.2 - Beta 4)]

I am unable to run 1.5.2 beta 4.  I get the "A purchased copy of Affinity Photo must be installed to use this beta." message, and then it quits.  Version 1.5.1 is installed; I get the message whether it's running or not.   iMac (27 inch, late 2012), running OS X 10.12.3. 

 

I have not been able to determine whether my Library Panel problem has been solved in the beta version.  This problem is copied below from the mac problems forum:

 

I cannot view the library panel on my iMac (27 inch, late 2012), running OS X 10.12.3.  That is in spite of downloading a fresh copy of Affinity Photo 1.5.1 from the App Store and following your Reset instructions posted 10 December 2016.  There is a View-> Studio -> Library entry, but checking/unchecking that entry has no effect.  Re-starting Affinity Photo did not help.

 

In contrast, I can view the library panel (and install the macro pack) on my MacBook Air (13 inch, mid 2011), also running OS X 10.12.3. 

[User Interface Deficiencies]

Please, folks, work on the user interface.  Just making the sizes of the icons in the tool dock adjustable would make a substantial difference. I confess, I was really surprised to find that ability was not there. The default list of tools on the left side of the window (a) takes up only about half the space on my 27" imac monitor, and (b) is impossible to read without my leaning over and squinting.  I'm 79 and my eyeballs are not in great shape, and no doubt I'll die pretty soon, so please hurry.